How to determine a reasonable attachment size limit?

I'll confess this is an area where I'll give it 10-20mb and toss out an "email is not intended for file transfer" whenever a user gripes about having to use FTP.

But a shiny new mail server deserves a rational approach... so what is a non-voodoo method for determining an appropriate limit for attachment size?

(Wavering on whether this is a wiki, or if there's a method that's Just That Good.)

I thought there would be some good guidelines independent of environment, but specifics were called for - so 50 mailboxes, exchange 2007, AD, hardware is TBD. Clients are a 2007/2003 mix, I figured I'd set sent/received to match, just to keep things simple.

Solution 1:

"Email is not intended for file transfer!"

In all seriousness, I set mine at 10MB, any higher and you might get rejections from remote SMTP servers. If your company/client does use a lot of larger files, I might be convinced to set it at 15 or 20MB but no higher than that.

I instruct clients to use a service like Dropbox if sending larger files. [Disclosure, that is my referral link!]

Solution 2:

The limit itself is somewhat less important than providing a consistent, secure and easy-to-use alternative to users who need to send and receive larger files.

Solution 3:

This is directly dependent on your business.

I have users who routinely get files in the 40MB range, and sometimes well above that. I essentially set an unlimited size for that reason.

Take a look at your legitimate attachments, take the average size and double it, then look at the largest legitimate attachment you've received. If it's bigger than double the average, then make it 50% bigger than the largest so far.